1. Field of the Invention
The present invention relates generally to an optical surface measuring apparatus and method, and, more particularly, to an apparatus and method which is capable of accurately measuring surface status, such as a minute variation in height (the height difference), a protrusion, a depression, surface damage and/or surface roughness, at each point on the surface of the object to be measured in an optical manner.
2. Description of the Related Art
In general, when optical parts, wafers or glass products are manufactured, Fizeau interferometers and point-diffraction interferometers based on the principle of interference are used to measure the shapes of manufactured products.
Such optical measuring apparatuses using interferometers can accurately measure variation in height (the height difference) of the surface of an object to be measured. However, in general, the height can be measured up to ¼ of wavelength λ due to interference without use of any correction.
For example, in the case of a helium-neon laser with a wavelength of 632 nm, a critical point appears near about 150 nm, and from FIG. 7, it can be seen that a repetitive pattern appears at a height of about 150 nm. This phenomenon is referred to as 2π-ambiguity.
The relationship between phase and distance and the 2π-ambiguity of a reflective interferometer will be described in more detail below.
When a reference surface (the height difference reference=0 nm) is set on a specimen to be measured, such as that shown in FIG. 8, and measurement is performed using an interferometer, an interference signal is represented as a function of signal light and reference light, and may be generally expressed by the following Equation 1:IInterference=ISig+Iref+2√{square root over (IrefISig)} sin(Δφ+Φ)  (1)where ISig and Iref are the intensities of signal light and reference light (that is, |ESig|2 and |Eref|2), Φ is phase difference given by a system, which is a constant value, and Δφ is the optical path difference between signal light reflected from a surface of the specimen and reference light.
Furthermore, the relationship between the phase difference of the interference signal given by the optical path difference and the variation in the distance (the height difference) may be expressed by the following Equation 2:
                              Δ          ⁢                                          ⁢          φ                =                                            2              ⁢              π                        λ                    ⁢          n          ⁢                                          ⁢          Δ          ⁢                                          ⁢          L                                    (        2        )            
When the variation on the surface of the specimen, that is, the height difference, is measured using the reflective interferometer, the refractive index of air is 1, and the optical path difference is doubled because the relative phase difference causes two optical path differences to be detected both when a laser beam enters into the surface and when the laser beam exits from the surface.
That is, since in FIG. 8, a point where there is no height difference causes the optical path difference to occur both when a laser beam enters into the reference surface and when the laser beam exits from the reference surface, unlike a point where there is the height difference, the resulting optical path difference measured by an actual reflective interferometer is twice the actual height difference.
Accordingly, in the case of the reflective interferometer, the phase variation is doubled, so that variation in the distance (or height difference) based on the phase difference is expressed by the following Equation 3:
                              Δ          ⁢                                          ⁢          L                =                              λ                          4              ⁢              π                                ⁢          Δφ                                    (        3        )            
Furthermore, FIG. 9 is a drawing showing an interference signal (values obtained by measuring an interferometer signal using an oscilloscope). From FIG. 9(1), it can be seen that the interference signal is repeated in periods of 2π because the interference signal has a sine wave form. As a result, in the case of the reflective interferometer, the maximum value of Δφ is π, so that the maximum variation in the distance is λ/4. This phenomenon is referred to as 2π-ambiguity.
When the interference signal measured using the interferometer and shown in FIG. 9(1) is calculated using a phase-distance variation equation, it can be represented by the variation in the distance, as shown in FIG. 9(2). FIG. 9(2) shows the values obtained by converting the interferometer signal into time-dependent variation in the distance through calculation.
In general, when the distance between points of measurement on a specific specimen is minimized, the height difference can be decreased indirectly. Accordingly, using this method, a case in consideration is regarded as the case where variation in the distance is equal to or leas than λ/4, so that the variation in the distance, such as that shown in FIG. 9(2), can be unwrapped using a program, as shown in FIG. 9(3).
This process is referred to as phase unwrapping. However, although a very narrow measuring interval is maintained, only variation equal to or less than λ/4 can be detected when measurement is performed at a point where variation in the distance (or height difference) is abruptly changed to a value equal to or greater than λ/4. That is, there is no method of finding the sequential position of a corresponding period.
Currently, various studies and methods have been conducted or proposed to overcome the above-described 2π-ambiguity. Of these methods, a method capable of measuring the longest distance has a maximum measuring range of 10 μm.
A conventional method capable of measuring a maximum of 10 μm will be described briefly below with reference to FIG. 10.
When the quantities of light are measured at the points of measurement, the intensities of light appear as shown in the left view of FIG. 10. When the linear interval is regularly performed, a line having a constant slope passes through a repetitive interference signal, in which case interference can be detected after the number of a point where the same intensity of light is measured has been repeated a predetermined number of times.
However, this conventional method is advantageous in that it is inconvenient to actually use because it is impossible to perform measurement at the focal point, and the maximum range of measurement is about 10 μm.
Furthermore, the case where a distance is measured using a focus error signal (which may be referred to as the “FES” or “FE signal”) can have a wide measurable range when the slope is moderate as shown in the left view of FIG. 11. However, a disadvantage arises in that the intensity of a signal generated at every distance is relatively weakened. When the signal is weakened as described above, the Signal Noise Ratio (SNR) is decreased, thus resulting in low resolution.
In contrast, when the slope is steep as shown in the right view of FIG. 11, the intensity of a signal generated at every distance is relatively strengthened, so that the resolution is high. However, a disadvantage arises in that a measurable range is rapidly decreased.
As a result, when a focus error signal is used for a distance measuring apparatus, only an apparatus which has high resolution and is capable of measuring only short intervals or an apparatus which has low resolution and is capable of measuring long intervals can be fabricated.
The above-described conventional method of measuring distance using an FE signal is disadvantageous in that the measurable range and corresponding resolution are inversely related to each other. The conventional method using interference has a high resolution up to the sub-nano level, but can measure about 160 nm corresponding to ¼ of laser wavelength at a focal point in the case of the 650 nm laser which is commonly used. Furthermore, although one of the above-described conventional methods is capable of measuring a maximum of 10 μm, this method has the disadvantage of being incapable of measurement at a focal point.